The present invention relates to a photovoltaic device and a process for production of said photovoltaic device, said process employing a diffusion protecting plate.
Production of photovoltaic devices involves the diffusion of an impurity. One process for diffusion is disclosed in, for example, xe2x80x9cSolar Cellsxe2x80x9d by Martin A. Green, issued by Prentice-Hall Inc., p. 109, FIGS. 6.4, 6.5(a), and 6.5(b). According to this process, a plurality of semiconductor substrates are arranged at certain intervals in a quartz furnace tube so that the semiconductor substrates are uniformly exposed to an impurity gas. In this process, the impurity is phosphorus and the semiconductor substrate is silicon, and the diffusion to make an impurity layer that exhibits the n-conductivity type is carried out by heating at 800-900xc2x0 C. The disadvantage of this process is that the diffusion of the impurity takes place on both sides of the semiconductor substrate as shown in FIG. 6.5(a) cited above. (This figure corresponds to FIG. 18C in the present specification.)
In the case where it is desirable to form the impurity diffusion layer on only one side of the semiconductor substrate, as shown in FIG. 6.5(b) cited above, the conventional process for impurity diffusion suffers a disadvantage of requiring a complex procedure. That is, it is necessary to previously form, prior to diffusion, an anti-diffusion film 14 on one side of the semiconductor substrate, as shown in FIG. 18B. This anti-diffusion film is a thermal CVD nitride film or thermal oxide film or a nitride film or oxide film formed at low temperatures by plasma CVD. Alternatively, it is necessary to diffuse an impurity into both sides of the semiconductor substrate and subsequently remove the diffusion layer on one side by etching or mechanical grinding. These additional steps for anti-diffusion film and etching increase the production cost of photovoltaic devices. Moreover, heat treatment at high temperatures for thermal oxidation deteriorates the quality of the semiconductor substrate, for example, decreasing the lifetime of minority carriers. Also, heat treatment brings about impurity redistribution diffusion, thereby altering the impurity profile. The resulting photovoltaic devices are poor in photoelectric conversion efficiency due to improper impurity profiles. In addition, etching and mechanical grinding change the surface state of the semiconductor substrate. The resulting photovoltaic devices are poor in photoelectric conversion efficiency because the surface texture to prevent light reflection is deformed.
In order to solve the above-mentioned problems encountered in the conventional technology, the present inventors carried out a series of researches. As the result, it was found that the problems are solved if impurity diffusion is carried out according to any of the following processes.
A process in which a first stage of impurity diffusion is carried out in such a way that at least one semiconductor substrate and at least one diffusion protecting plate are put close to each other.
A process in which a first stage of impurity diffusion is carried out in such a way that at least one semiconductor substrate and at least one diffusion protecting plate are put close to each other and subsequently a second stage of impurity diffusion is carried out in such a way that said semiconductor substrate and said diffusion protecting plate are put close to each other, with the side which has undergone diffusion facing inside.
A process in which the diffusion protecting plate is a semiconductor substrate.
A process in which impurities of the same conductivity type are used in the first and second stages of impurity diffusion.